Charge equalization system for batteries

ABSTRACT

An equalization system for batteries has two series-connected accumulator stages connected, each including an accumulator, a voltage generator including positive and negative poles, for each stage, an associated charging device supplied by the voltage generator. The charging device includes an inductor having a first and second ends, and a capacitor having first and second ends. The capacitor&#39;s first end connects to the generator&#39;s positive pole. It also has a diode whose anode connects to a negative pole of the accumulator stage and whose cathode connects to the inductor&#39;s first end. It also has a switch whose first end connects to the inductor. And it also has a control device that controls the generator and closes a switch of a charging device associated with an accumulator stage to be charged so that the inductor stores energy, and causes its transfer to that accumulator stage.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is the National Stage of International Application No. PCT/EP2011/051688, filed on Feb. 4, 2011, which claims the benefit of the priority date of French Application No. 1000478, filed on Feb. 5, 2010. The content of these applications is hereby incorporated by reference in its entirety.

FIELD OF DISCLOSURE

The invention concerns a charge equalization system for electrochemical accumulator batteries, usable notably in the field of electric or hybrid transport and onboard systems. The invention concerns in particular batteries of lithium-ion (Li-ion) type suitable for this kind of application because of their ability to store large amounts energy with a low mass. The invention is equally applicable to supercapacitors.

BACKGROUND

An electrochemical accumulator has a nominal voltage of the order of a few volts, to be more precise 3.3 V for Li-ion batteries based of iron phosphate and 4.2 V for a Li-ion technology based on cobalt oxide. If this voltage is too low relative to the requirements of the systems to be supplied with power, a plurality of accumulators are connected in series. It is equally possible to dispose one or more accumulators in parallel with each series-connected accumulator in order to increase the available capacity and to provide a higher current and a higher power. The parallel-connected accumulators then form what is referred to herein as a stage. A stage is constituted of at the minimum one accumulator. The stages are connected in series to reach the required voltage level. The association of accumulators is called an accumulator battery.

Charging and discharging an accumulator is reflected in an increase or a decrease, respectively, in the voltage at its terminals. An accumulator is considered charged or discharged when it reaches a voltage level defined by the electrochemical process. In a circuit utilizing a plurality of accumulator stages, the current in the stages is the same. The level of charge or discharge of the stages thus depends on the intrinsic characteristics of the accumulators, namely the intrinsic capacitance and the series and parallel parasitic internal resistances of the electrolyte or of contact between the electrodes and the electrolyte. Voltage differences between the stages are then possible because of fabrication disparities and ageing.

For a Li-ion technology accumulator, too high or too low a voltage, called the threshold voltage, may damage or destroy the accumulator. For example, overcharging a Li-ion accumulator based on cobalt oxide may lead to thermal runaway and start a fire. For a Li-ion accumulator based on iron phosphate, overcharging is reflected in decomposition of the electrolyte that decreases its service life or can damage the accumulator. Too great a discharge, for example leading to a voltage less than 2 V, leads primarily to oxidation of the current collector of the negative electrode if it is made of copper and thus to damage to the accumulator. Consequently, surveillance of the voltages at the terminals of each accumulator stage is obligatory during charging and discharging for reasons of safety and reliability. A so-called surveillance device in parallel with each stage enables this function to be provided.

The surveillance device has the function of tracking the state of charge and discharge of each accumulator stage and transmitting the information to the control circuit in order to stop charging or discharging the battery if a stage has reached its threshold voltage. However, in the case of a battery with a plurality of accumulator stages disposed in series, if charging is stopped when the stage with the highest charge reaches its threshold voltage, the other stages may not be fully charged. Conversely, if discharge is stopped when the most discharged stage reaches its threshold voltage, the other stages may not be totally discharged. The charge in each accumulator stage is thus not used optimally, which represents a major problem in transport and onboard type applications having severe autonomy constraints. To alleviate this problem, the surveillance device is generally associated with an equalization device.

The equalization device has the function of optimizing the charge of the battery and therefore its autonomy by bringing the accumulator stages connected in series to an identical state of charge and/or discharge. There exist two categories of equalization devices, so-called energy dissipation equalization devices and so-called energy transfer equalization devices.

In the case of energy dissipation equalization devices, the voltage at the terminals of the stages is rendered uniform by bypassing the charging current of one or more stages when the threshold voltage has been reached and dissipating the energy in a resistor. Alternatively, the voltage at the terminals of the stages is rendered uniform by discharging one or more stages where the threshold voltage has been reached. However, such energy dissipation equalization devices have the major drawback of consuming more energy than is necessary to charge the battery. This circuit makes it obligatory to discharge a plurality of accumulators or to divert the charging current of a plurality of accumulators so that the last accumulator or accumulators with a slightly lower charge finish(es) charging. The energy dissipated can therefore be very much greater than the charge or charges to be completed. Moreover, the excess energy is dissipated as heat, which is not compatible with integration constraints in transport and onboard type applications, and the service life of the accumulators falls sharply if the temperature is raised.

Energy transfer equalization devices exchange energy between the accumulator battery or an auxiliary power network and the accumulator stages.

There is known from the U.S. Pat. No. 5,659,237, for example, a device enabling transfer of energy from the auxiliary network to the stages by a “flyback” structure with a plurality of outputs using a coupled inductor as a storage element. The latter element is a specialized component because it is dedicated to this application. The cost of such a component is prohibitive for the function to be implemented.

There is known from the patent CN1905259 a device enabling transfer of energy from the stages to the battery and that for its part uses an inductor for each accumulator as a storage element. However, this device does not opt for optimized energy transfer for equalization of the batteries in transport and onboard type applications. The end of charging of a battery is indeed determined by the last stage to reach the threshold voltage. To terminate the charging of the battery, energy is taken from one or more stages and returned to the group of stages. If one or more accumulator stage(s) is or are slightly less charged, energy is then not transferred with priority to the latter stage(s) that require(s) it but also to the stage(s) from which the energy is taken. Equalization thus necessitates taking energy from all stages at the end of charging in order to prevent charging them to too high a voltage. Equalization is thus effected with high losses because of the large number of converters operating. Moreover, accumulators for which charging has already ended have alternating or direct current components of no utility pass through them.

An object of the invention is therefore to propose an improved equalization device that does not have these drawbacks of the prior art.

SUMMARY

To this end, the invention provides an equalization system for batteries comprising at least two accumulator stages connected in series, each accumulator stage comprising at least one accumulator, characterized in that said system includes:

-   -   at least one voltage generator comprising at least one positive         pole and at least one negative pole,     -   for each accumulator stage an associated charging device fed by         said at least one voltage generator and comprising:         -   at least one inductor,         -   at least one capacitor the first end of which is connected             to said positive pole of said at least one voltage             generator,         -   at least one diode connected by its anode to the negative             pole of said accumulator stage and by its cathode to the             first end of said at least one inductor, and         -   at least one switch one end of which is connected to one end             of said at least one inductor, and     -   a control device configured to control said at least one voltage         generator and to close said at least one switch of a charging         device associated with a accumulator stage to be charged, so         that said at least one inductor stores energy, and to transfer         that energy to said associated accumulator stage.

Said equalization system may further include one or more of the following features, separately or in combination:

-   -   the second end of said at least one capacitor is connected to         the first end of the inductor, the cathode of said at least one         diode is connected to the second end of the capacitor, and said         at least one switch is connected by its first end to the second         end of the inductor and by its second end to the positive pole         of the associated accumulator stage;     -   said charging device includes at least one first capacitor the         first end of which is connected to said positive pole of the         voltage generator and the second end of which is connected to         the first end of the inductor, at least one second capacitor the         first end of which is connected to said negative pole of said         voltage generator and the second end of which is connected to         the negative pole of the associated accumulator stage, said at         least one diode is connected by its anode and its cathode to the         second end of said second capacitor and to the second end of         said first capacitor, respectively, and said at least one switch         is connected by its first end to the second end of the inductor         and by its second end to the positive pole of the associated         accumulator stage;     -   said charging device includes at least one first capacitor the         first end of which is connected to said positive pole of the         voltage generator, at least one second capacitor the first end         of which is connected to said negative pole of said voltage         generator and the second end of which is connected to the second         end of the inductor and to the positive pole of the associated         accumulator stage, and said at least one switch is connected by         its first end to the second end of the first capacitor and by         its second end to the first end of the inductor;     -   said charging device includes at least one first inductor and         one second inductor, at least one first capacitor the first end         of which is connected to said positive pole of a voltage         generator and the second end of which is connected to the first         end of the first inductor, at least one second capacitor the         first end of which is connected to said negative pole of said         voltage generator and the second end of which is connected to         the first end of the second inductor, at least one first diode         the anode and the cathode of which are connected to the negative         pole of the associated accumulator stage and to the first end of         the first inductor, respectively, at least one second diode the         anode and the cathode of which are connected to the negative         pole of the associated accumulator stage and to the first end of         the second inductor, respectively, and said at least one switch         is connected by its first end to the second ends of said         inductors and by its second end to the positive pole of the         associated accumulator stage;     -   the control device is configured to control the rate of closing         of said at least one switch of a charging device associated with         a accumulator stage to be charged when a voltage is applied         beforehand to the poles of said voltage generator feeding said         charging device;     -   said charging device is configured to operate in discontinuous         conduction mode independently of the voltages of the associated         accumulator stage and the battery during the charging phase;     -   said equalization system includes single voltage generator for         feeding all of said charging devices;     -   said battery equalization system includes a voltage generator         for each charging device;     -   said battery equalization system includes at least two voltage         generator devices associated with a respective predetermined         number of charging devices;     -   said at least one voltage generator includes at least one switch         controlled by the control device;     -   said at least one voltage generator includes a decoupling         capacitor;     -   said at least one voltage generator includes two controlled         switches and two inductors;     -   said at least one voltage generator includes a switch and a         transformer;     -   said at least one voltage generator includes a bridge including         four switches and a transformer;     -   said at least one voltage generator includes two switches, a         transformer and two capacitors;     -   said at least one voltage generator includes two switches and a         transformer with a center-tapped primary;     -   the control device is configured to close and to open said at         least one controlled switch of said at least one voltage         generator device with a conduction period and an opening time,         respectively, that are constant during a charging phase;     -   the conduction period is calculated so that said charging device         operates in discontinuous conduction mode;     -   said at least one voltage generator is connected to the         terminals of the battery;     -   said battery includes at least one elementary module, each         elementary module comprising a plurality of accumulator stages         in series, and said equalization system further includes a         supplementary charging device at the terminals of each         elementary module;     -   said battery includes a plurality of elementary modules disposed         in series and said equalization system includes a supplementary         charging device at the terminals of a predetermined number of         elementary modules;     -   said at least one voltage generator is connected to the         terminals of said at least one elementary module;     -   said battery equalization system includes a device for measuring         the voltage of each accumulator stage, configured to transmit         voltage information to the control device;     -   the accumulators are of lithium-ion type;     -   the battery includes supercapacitors.

The invention also provides a charging device for a charge equalization system as defined above.

BRIEF DESCRIPTION OF THE FIGURES

Other features and advantages of the invention will become more clearly apparent on reading the following description, given by way of illustrative and nonlimiting example, and from the appended drawings, in which:

FIG. 1 is a block diagram of a battery including accumulator stages connected in series and a battery charge equalization system including a charging device for each accumulator stage and a voltage generator,

FIG. 2 is a block diagram of a variant of the equalization system from FIG. 1 including a voltage generator for each charging device,

FIG. 3 is a block diagram of another variant of the equalization system from FIG. 1 including a voltage generator for each elementary module including a predetermined number of accumulator stages connected in series,

FIG. 4 is a block diagram of another variant of the equalization system from FIG. 3 including a supplementary charging device for each elementary module,

FIG. 5 is a block diagram of a first embodiment of a charging device of the equalization system,

FIG. 6 is a block diagram of one exemplary embodiment of a voltage generator associated with the charging device from FIG. 5 in continuous conduction mode,

FIG. 7 is a block diagram of one exemplary embodiment of a voltage generator associated with the charging device from FIG. 5 in discontinuous conduction mode,

FIG. 8 a is a block diagram of the equalization system including the charging device from FIG. 5 and the voltage generator from FIG. 6, in which there has been indicated the flow of the current during a conduction period,

FIG. 8 b is a block diagram of the equalization system including the charging device from FIG. 5 and the voltage generator from FIG. 6, in which there has been indicated the flow of the current at the end of the conduction period and until blocking of the diode of the charging device,

FIG. 8 c is a block diagram of the equalization system including the charging device from FIG. 5 and the voltage generator from FIG. 6, in which there has been indicated the flow of the current once the diode of the charging device is blocked,

FIG. 9 is a diagram representing the evolution as a function of time of the various currents in the charging device from FIG. 5 and in the parallel accumulator stage,

FIG. 10 shows diagrammatically curves of evolution of the various currents in the charging device from FIG. 5 and in the associated accumulator stage for a first simulation,

FIG. 11 shows diagrammatically curves of evolution of the various currents in the charging device from FIG. 5 and in the associated accumulator stage for a second simulation,

FIG. 12 is a block diagram of a second embodiment of a charging device of the equalization system,

FIG. 13 is a block diagram of one exemplary embodiment of a voltage generator associated with the charging device from FIG. 12,

FIG. 14 is a block diagram of a variant embodiment of the voltage generator from FIG. 13,

FIG. 15A is a block diagram of the equalization system including the charging device from FIG. 12 and the voltage generator from FIG. 13, in which there has been indicated the flow of current during the conduction period,

FIG. 15B is a block diagram of a circuit including the charging device from FIG. 12 and the voltage generator from FIG. 13, in which there has been indicated the flow of current at the end of the conduction period until the operating period,

FIG. 16 is a diagram representing the evolution as a function of time of the various currents in the charging device from FIG. 12 and in the parallel accumulator stage,

FIG. 17 shows diagrammatically curves of evolution of the various currents in the charging device from FIG. 12 and in the associated accumulator stage,

FIG. 18 is a block diagram of a third embodiment of a charging device of the equalization system,

FIG. 19A is a block diagram of the equalization system including the charging device from FIG. 18 and the voltage generator from FIG. 13, in which there has been indicated the flow of the current during the conduction period,

FIG. 19B is a block diagram of the equalization system including the charging device from FIG. 18 and the voltage generator from FIG. 13, in which there has been indicated the flow of the current at the end of the conduction period until the operating period,

FIG. 20 shows diagrammatically curves of evolution of the various currents in the charging device from FIG. 18 and in the associated accumulator stage,

FIG. 21 is a block diagram of a fourth embodiment of a charging device of the equalization system,

FIG. 22A is a block diagram of a first variant embodiment of the charging device from FIG. 21,

FIG. 22B is a block diagram of a second variant embodiment of the charging device from FIG. 21,

FIG. 23 is a block diagram of one embodiment of a voltage generator associated with the charging device from FIGS. 21, 22A and 22B,

FIG. 23′ is a block diagram of a variant embodiment of the voltage generator from FIG. 23,

FIG. 24A is a block diagram of a variant embodiment of the voltage generator from FIG. 23,

FIG. 24B is a block diagram another variant embodiment of the voltage generator from FIG. 23,

FIG. 25A is a block diagram of the equalization device including the charging device from FIG. 21 and the voltage generator from FIG. 24A, in which there has been indicated the flow of the current during the conduction period,

FIG. 25B is a block diagram of the equalization device including the charging device from FIG. 21 and the voltage generator from FIG. 24A, in which there has been indicated the flow of current at the end of the conduction period and up to half of the operating period,

FIG. 26 is a diagram representing diagrammatically the evolution as function of time of the various currents in the charging device from FIG. 21,

FIG. 27 shows diagrammatically curves of evolution of the various currents in the charging device from FIG. 21 and in the associated accumulator stage for a first simulation, and

FIG. 28 shows diagrammatically curves of evolution of the various currents in the charging device from FIG. 21 and in the associated accumulator stage for a second simulation.

In these figures, substantially identical elements bear the same references.

DETAILED DESCRIPTION

FIG. 1 shows an accumulator battery 1. This battery 1 is composed of N stages Et_(i) connected in series. Each stage Et_(i) is composed of one accumulator or a plurality of accumulators A_(ij), connected in parallel. Here the suffix i represents the stage number, and this suffix i varies in the example shown in FIG. 1 from 1 to N, and the suffix j represents the number of each accumulator in a given stage, and in the example shown this suffix j varies from 1 to M. The terminals of the accumulators A_(ij) of the same stage Et_(i) are connected together by means of electrical connections, just as each stage Et_(i) is also connected to the adjacent stages Et_(i) by means of electrical connections.

The subject of the invention is a charge equalization system 2 for a accumulator battery 1 of this kind including at least two stages Et_(i) connected in series.

The equalization system 2 further includes a control device 3, a plurality of identical charging devices 5 for each accumulator stage Et_(i), and at least one voltage generator 7 as shown in FIG. 1. The charging devices 5 and the voltage generator(s) 7 are controlled by the control device 3.

The equalization system 2 may further include a voltage measuring device (not shown) for measuring the voltage of each stage Et_(i) and for transferring voltage information to the control device 3, which is able on the basis of this voltage information to determine if a accumulator stage Et_(i) must be charged and to control accordingly the charging device 5 in parallel with the accumulator stage and the associated voltage generator 7.

Each accumulator stage Et_(i) is associated with one of the charging devices 5 and with a single voltage generator 7, for example.

The charging devices 5 are connected on the one hand to the negative pole N_(i) and to the positive pole P_(i) of each accumulator stage Et_(i) and on the other hand to the positive pole, denoted v2, and to the negative pole, denoted v1, of one or more voltage generators 7.

In the case of a single voltage generator 7 (FIG. 1), the latter is connected to the group of charging devices 5.

In the case of multiple voltage generators 7, a voltage generator 7 is connected to a charging device 5 if the number of voltage generators 7 is equal to the number of stages Et_(i), as shown by way of example in FIG. 2.

In an alternative shown in FIG. 3, a voltage generator 7 may be connected to a plurality of charging devices 5 if the number of voltage generators 7 is less than the number of stages Et_(i).

For example, if a large number of accumulator stages Et_(i) in series is used, as is the case for electric vehicles with one hundred accumulators in series, for example, the battery 1 may be constituted by the series connection of elementary modules 9 (FIG. 3), each including for example ten to twelve accumulator stages Et_(i) connected in series. Thus the voltage generator(s) 7 is (are) connected to the terminals of ten to twelve elements. The withstand voltage of the diodes and the controlled switches is limited, as a function of the Li-ion battery technology, to approximately 45 V-60 V, which is a standardized withstand voltage value in the semiconductor field. Maintenance of a large number of elementary modules 9, as is the case for electric vehicles, is facilitated.

Moreover, in addition to the charging devices 5 for the accumulator stages Et_(i), identical charging devices 5 may be used by connecting N stages in series as shown in FIG. 4. This variant enables transfer of energy between the N adjacent stages and thus between the associated elementary modules 9 in series. In this case, one or more supplementary voltage generators 7 supply energy to the charging devices 5 connected to the terminals of N stages.

The voltage generator(s) 7 supplies (supply) the charging devices 5 with voltage pulses of positive, negative or positive and negative polarity and of varied shape, for example squarewave or sinusoidal.

First Embodiment

A first embodiment of a charging device 5 and a voltage generator 7 of the equalization system 2 is described next.

Charging Device

Referring to FIG. 5, in the first embodiment a charging device 5 includes:

-   -   an inductor L1 _(i),     -   a capacitor C1 _(i) the first end of which is connected to the         pole v2 of a voltage generator 7 and the second end of which is         connected to the first end of the inductor L1 _(i),     -   a diode D1 _(i) the anode and the cathode of which are connected         to the pole N_(i) of the stage and to the second end of the         capacitor C1 _(i), respectively,     -   a switch SW1 _(i), for example a MOSFET, the first end of which         is connected to the second end of the inductor L1 _(i) and the         second end of which is connected to the pole P_(i) of the stage.

The control device 3 enables opening and closing of the switch SW1 _(i).

If the pole v1 of the voltage generator 7 is connected to the negative pole N of the battery 1 (FIGS. 1 to 4), the charging device 5 of this first embodiment has the particular feature of being connected to the pole v1 of the voltage generator 7 via the accumulator stage Et_(i) to which the charging device 5 is connected and also via the accumulator stages situated below this stage Et_(i).

A charging device 5 of this kind operates either in continuous conduction mode or in discontinuous conduction mode.

Operation in discontinuous conduction mode is preferred because it has the advantage of being of lower cost and easier to implement.

In discontinuous conduction mode, the current in the inductor L1 _(i) is indeed by definition cancelled before each period T of operation of the charging device 5. The value of the current in the inductor L1 _(i) when the voltage generator 7 is supplying energy can be deduced from the voltage applied to the terminals of the inductor L1 _(i), the time of storage of energy in the inductor L1 _(i) and the value of the latter. After this, the voltage generator 7 may be controlled by a fixed conduction period command.

On the other hand, in continuous conduction mode (FIG. 6), it would be necessary to use a current sensor 10 associated with a regulation loop 11 and with a current reference variable 13, as well as a device for monitoring the current 12, for example in a switched mode using transistors operating as switches, for each of the accumulator stages in series. In continuous conduction mode, a voltage generator 7 would be necessary for each charging device 5. The voltage generator 7 would then be controlled in a pulse width modulation mode.

In discontinuous conduction, the present invention enables all the charging devices 5 to be connected in parallel to the output of a voltage generator 7 thanks to the blocking of direct current by the capacitors C1 _(i).

In a variant embodiment, a capacitor (not shown) may be added at the output of each charging device 5. This capacitor is connected between the terminal N_(i) and the first end of the switch SW1 _(i). A capacitor of this kind is configured to filter the ripple on the current coming from the charging device 5. A smoothed direct current is thus supplied to each accumulator stage when the charging device 5 is operating.

The position of the switch SW1 _(i) may also be modified to connect it in series with the capacitor C1 _(i).

Two other positions of the switch SW1 _(i) are possible: the switch SW1 _(i) may be connected either to the output v2 of the voltage generator 7 and to the first end of the capacitor C1 _(i) or to the second end of the capacitor C1 _(i) and to the cathode of the diode D1 _(i).

Another variant embodiment consists in using for each charging device 5 a controlled switch instead of a diode. So-called synchronous rectification is then possible. The efficiency of the charging device 5 may be increased thanks to the reduction in the voltage drop in the on state of the component.

Voltage Generator

The voltage generator 7 may have varied structures. One embodiment suitable for a voltage generator 7 supplying only a positive voltage pulse, for example.

FIG. 6 shows an embodiment of a voltage generator 7 associated with the charging device 5 of the first embodiment.

It includes two switches SW2 _(i) and SW3 _(i), for example MOSFETs, connected to the terminals N and P of the battery 1. A so-called decoupling capacitor C3 _(i) may be connected to the terminals of the two switches SW2 _(i) and SW3 _(i).

The voltage generator 7 may be connected to the terminals of an elementary module 9 but equally to the terminals of the battery 1 or even to the terminals of an auxiliary supply (vehicle 12 V supply, for example).

Operation

The operation of the first embodiment is described hereinafter with reference to FIGS. 8 a to 8 c and 9.

The charging devices 5 enable the charging of certain stages under charge to be continued. Operation in discontinuous conduction mode is preferred for the reasons described above.

For example, if the control device 3 commands the transfer of energy to a stage Et_(i), for example the stage Et₁, the switch SW1 ₁ of the charging device 5 in parallel with the corresponding stage Et_(i) is closed by the control device 3.

The voltage generator 7 supplying power to the charging device 5 is also activated by the control device 3.

The stages in series with the stage Et₁ are not charged while the switch SW1 ₁ of the parallel-connected charging devices 5 of each stage remains in the open state.

If a charging device 5 begins to operate and the voltage generator 7 was operating beforehand, the speed of closing the switch SW1 ₁ must be controlled in order to prevent too high a current being supplied to the stage.

The switches are considered as perfect when they are in the blocked state and therefore allow no current to flow when they are in this state.

First Variant: Voltage Generator with No Decoupling Capacitor

Consider the voltage generator 7 from FIG. 6 without the decoupling capacitor C3 _(i) at the terminals of the switches SW3 _(i) and SW2 _(i).

Referring to FIGS. 8 a to 8 c and 9, during a conduction period t1, a positive voltage is applied between the terminals v2 and v1 of the voltage generator 7. The switch SW2 ₁ is closed and the switch SW3 ₁ is open. The voltage generator 7 therefore supplies a positive voltage pulse while the switch SW2 ₁ is closed and the switch SW3 ₁ is open.

The flow of current during the time t1 is shown diagrammatically in FIG. 8 a by dotted line arrows.

Energy is stored in the inductor L1 ₁. A capacitor C1 ₁ of sufficiently high value is necessary to transmit sufficient current and to impose a quasi-constant voltage at the terminals of the inductor L1 ₁. The current in the inductor L1 ₁ increases proportionately to the voltage applied to its terminals, approximately equal to the voltage of the battery 1 less the voltage of the stage Et₁ under charge. The current flows through the stages situated above the stage Et₁. The diode D1 ₁ is blocked during this phase. The voltage at the terminals of the diode D1 ₁ is equal to minus the voltage of the battery 1.

At the end of the time t1, the output voltage of the voltage generator 7 becomes zero. The switch SW2 ₁ of the voltage generator 7 opens. The switch SW3 ₁ closes when the switch SW2 ₁ is fully open.

The current in the inductor L1 ₁ reaches at this time a peak value Ipic (FIG. 9) approximately equal to the voltage applied to the terminals of the inductor when the voltage generator 7 is supplying energy, multiplied by t1 and divided by the value of the inductor. This formula is approximate in that it considers that the current in the inductor is zero before each period of operation of the charging device 5.

The charging device 5 has the particular feature that the current in the inductor L1 ₁ goes negative and thereafter oscillates between negative and positive values when the diode D1 ₁ is blocked, and thus before each period T of operation of the charging device 5. This particular feature is a necessity in that the mean current through the capacitor C1 ₁ must be zero for each period T of operation of the charging device 5 in the permanent regime.

At the end of the time t1 and until the diode D1 ₁ is blocked, the voltage generator 7 imposes a zero voltage at the terminals v2 and v1. The switch SW3 ₁ is closed and the switch SW2 ₁ is open. The flow of the current during this phase is shown diagrammatically in FIG. 8 b by chain-dotted line arrows.

The current in the inductor L1 ₁ decreases proportionately to the voltage applied to its terminals, equal to minus the voltage of the accumulator stage Et₁ less the voltage drop in the on state of the diode D1 ₁. The diode D1 ₁ is on during this phase until the current in it is cancelled out. The flow of the current once the diode D1 ₁ is blocked is represented diagrammatically in FIG. 8 c by dashed-line arrows.

As explained above, the current in the inductor L1 ₁ becomes negative after conduction of the diode D1 ₁ and thereafter oscillates between negative and positive values (FIG. 9). The oscillation frequency of the current during this phase is approximately defined by the inherent resonant frequency of the capacitor C1 ₁ and the inductor L1 ₁. This current flows through the stage Et₁ under charge, the stages situated under the stage Et₁, the capacitor the inductor L1 ₁ and the switch SW3 ₁.

At the end of the period T, a new sequence of operation commences if the switch SW1 ₁ is still in the closed state. The voltage generator 7 is controlled in such a way that the two switches SW2 ₁ and SW3 ₁ are not on at the same time in order to prevent short-circuiting of the battery. Between the switch SW3 ₁ opening and the switch SW2 ₁ closing, it is imperative that the current in the inductor L1 ₁ is zero or positive to enable continuity of the current in the diode D1 ₁. The inherent resonant frequency of the capacitor C1 ₁ and the inductor L1 ₁ must be taken into consideration when dimensioning the device.

If the current in the inductor L1 ₁ should be negative, it would then be necessary for the switch SW2 ₁ to be bidirectional in terms of current flow, for example by using a diode connected in antiparallel with the switch SW2 ₁.

Second Variant: Voltage Generator with Decoupling Capacitor

In the context of the use of the decoupling capacitor C3 _(i) at the terminals of the switches SW3 _(i) and SW2 _(i) (FIG. 6), the circulation of the current is modified when the switch SW2 _(i) is closed. The capacitor C3 _(i) indeed also supplies part of the current that flows through the inductor L1 _(i). This current is looped by the stage Et_(i) under charge and by the stages situated under the stage Et_(i) under charge. The value of the current supplied depends on the position of the stage Et_(i) under charge relative to the N stages. The nearer the stage Et_(i) is to the terminal N of the battery, the greater the value of the current supplied by the capacitor C3 _(i) relative to the current that flows through the stages situated above the stage Et_(i) under charge.

Dimensioning

Equations

The dimensioning of the charging device 5 (FIG. 5) is determined using equations representing its operation as described above. The equations are effective for the associated voltage generator 7 (FIG. 6).

The use of equations as described hereinafter is generalized. To this end, the input and output voltages are respectively denoted Ve and Vs. The voltage Ve is the voltage between the terminals N and P of the battery 1. The voltage Vs is the voltage between the terminals N_(i) and P_(i) of an accumulator stage Et_(i).

To simplify dimensioning, the voltage at the terminals of the capacitor C1 _(i), if present, is considered constant over a period T of operation of the charging device 5.

During the time t1, the current (iL1 _(i)) in the inductor L1 _(i) increases. The current is considered zero in each period T of operation. Ignoring the voltage drop in the on state of the switches SW2 _(i) and SW1 _(i) and considering the voltage drop Vd with the diode D1 _(i) in the on state, the current iL1 _(i)(t) is written:

$\begin{matrix} {{{iL}\; 1_{i}(t)} = {\frac{{Ve} - {Vs} - {Vd}}{L\; 1_{i}} \times t}} & \left( {{equation}\mspace{14mu} 1} \right) \end{matrix}$

At the end of the time t1, the switch SW2 _(i) is opened. The current in the inductor reaches the following beak value:

$\begin{matrix} {{{iL}\; 1_{i}\left( {t\; 1} \right)} = {{Ipic} = {\frac{{Ve} - {Vs} - {Vd}}{L\; 1_{i}} \times t\; 1}}} & \left( {{equation}\mspace{14mu} 2} \right) \end{matrix}$

At the end of the time t1 and until the current in the inductor L1 _(i) is cancelled out, the diode D1 _(i) conducts. The current in the inductor L1 _(i), decreases in accordance with the following law.

$\begin{matrix} {{{iL}\; 1_{i}(t)} = {{{- \frac{{Vs} + {Vd}}{L\; 1_{i}}} \times t} + {Ipic}}} & \left( {{equation}\mspace{14mu} 3} \right) \end{matrix}$

The value (t1 _((max))) not to be exceeded of the conduction period t1 for the charging device 5 to function in discontinuous conduction mode can be defined from equation 3. The current in the inductor must be zero or positive for each period T of operation. The natural period of oscillation for the capacitor C1 _(i) and the inductor L1 _(i) is taken into account in the definition of equation 4. The time t1 _((max)) depends on two terms A and B. To take the worst case scenario, the term A is to be evaluated for the maximum input voltage Ve and the minimum output voltage Vs. The term B depends on the input and output voltages and on the mean output current (Is_(i(moy))). The term B is constant and can thus be evaluated either for the maximum input voltage and the minimum output voltage and the maximum output mean current or, conversely, for the minimum input voltage and the maximum output voltage and the minimum output mean current.

$\begin{matrix} {{{t\; 1_{(\max)}} = {T \times \frac{1}{\frac{1}{A} + B}}}{{{Where}\mspace{14mu} A} = \frac{{Vs} + {Vd}}{Ve}}{B = {\pi \times \sqrt{\frac{1}{2 \times T} \times \frac{C}{\left( {{Vs} + {Vd}} \right) \times {Is}_{i{({moy})}}}} \times \left( {{Ve} - {Vs} - {Vd}} \right)}}} & \left( {{equation}\mspace{14mu} 4} \right) \end{matrix}$

The output current of the charging device is equal to the current in the inductor L1 _(i). The mean output current of a charging device 5 is calculated from equation 5. Given that the mean current in the capacitor C1 _(i) is zero in the permanent regime, the mean output current (Is_(i(moy))) is equal to the mean current in the diode D1 _(i). The current Is_(i(moy)) is proportional to the input voltage Ve squared and inversely proportional to the output voltage Vs. To supply the desired mean current regardless of the voltage of the accumulator stage and for a given time t1, the maximum output voltage and the minimum input voltage must be taken into account.

$\begin{matrix} {{Is}_{i{({moy})}} = {{\frac{1}{2} \times \frac{1}{T} \times \frac{L\; 1_{i}}{\left( {{Vs} + {Vd}} \right)} \times {Ipic}^{2}} = {\frac{1}{2} \times \frac{1}{T} \times \frac{\left( {{Ve} - {Vs} - {Vd}} \right)^{2} \times t\; 1^{2}}{\left( {{Vs} + {Vd}} \right) \times L\; 1_{i}}}}} & \left( {{equation}\mspace{14mu} 5} \right) \end{matrix}$

The current iEt_(i)(t) in a charged stage Et_(i) depends on the number of charging devices 5 operating and their position relative to the stage Et_(i).

If only the charging device 5 in parallel with the stage Et_(i) is operating, the current in the stage Et_(i) is therefore equal to the current in the inductor L1 _(i) between t1 and T and is zero the rest of the time. This variant is represented in solid line in FIG. 9.

Considering the number L of charging devices 5 operating situated above the stage Et_(i), a supplementary current flows through the stage Et_(i) when the diode D1 _(i) of the corresponding charging devices is blocked. During this phase, and considering the charging devices to be identical, the current iEt_(i)(t) is equal to the current in a capacitor C1 _(i) multiplied by the number L of device(s) operating plus one. This variant is represented in dotted line in FIG. 9 for L=1.

Considering the number M of charging devices operating situated under the stage Et_(i), a supplementary current flows through the stage Et_(i) during the time t1. During this phase, and considering the charging devices to be identical, the current iEt_(i)(t) is equal and opposite the current in an inductor L1 _(i) multiplied by the number M of devices operating. This variant is represented in dashed line in FIG. 9 for M=1.

The mean value (IEt_((moy))) of the current in the charged stage is obtained from equation 6.

$\begin{matrix} {{IEt}_{i{({moy})}} = {\frac{1}{2} \times \frac{1}{T} \times {Ipic} \times \left\lbrack {\frac{{Ipic} \times L\; 1_{i}}{{Vs} + {Vd}} - {t\; 1 \times \left( {1 + L + M} \right)}} \right\rbrack}} & \left( {{equation}\mspace{14mu} 6} \right) \end{matrix}$

EXAMPLE

For example, each charging device 5 enables tracking of the charging of an accumulator stage Et_(i) and is connected to the terminals of ten stages. A single voltage generator 7 is considered for supplying all the charging devices. The latter generator is connected to the terminals of a series combination of ten stages.

The dimensioning process is divided into two steps. First, the maximum conduction period t1 of the switch SW2 _(i) of the voltage generator 7 is calculated for operation of the charging device in discontinuous conduction mode. Second, the maximum value of the inductor L1 _(i) for supplying the desired mean current at the output of the charging device is defined.

The following hypotheses apply:

mean output current (minimum, Is_(i(moy))): 1 A

typical operating frequency (F): 50 kHz, i.e. T=1/F=20 μs

voltage of one accumulator (Li-ion based on iron phosphate):

-   -   Minimum voltage: 2.5 V     -   Maximum voltage: 3.6 V

voltage drop (Vd) in on state of diodes: 0.3 V-0.7 V.

The capacitor C1 _(i) is dimensioned to transmit sufficient current and to impose a quasi-constant voltage at the terminals of the inductor L1 _(i) of the charging device 5. Considering a voltage drop of the order of 3 V, for a peak current of the order of 6 A and a time t1 of 2.0 μs, the value of the capacitor is of the order of 2 μF. The value of the capacitor is typically of the order of 1 μF to 10 μF.

The maximum values of the time t1 and the inductance of the inductor L1 _(i) are calculated hereinafter. The time t1 _((max)) is calculated using to calculate the term A the minimum voltage drop of the diode D1 _(i), the maximum input voltage and the minimum output voltage of the charging device and to calculate the term B, the minimum mean output current, the minimum input voltage and the maximum output voltage, and a blocking capacitor value set at 2 μF. The maximum inductance L1 _(i) is calculated using the same conditions as were used to calculate the term B from t1 _((max)). The time t1 _((max)) is used to calculate the inductance of the inductor L1 _(i) to minimize the peak current and the rms (root mean square) current in the inductor, the switches and the diodes. For reasons of robustness, however, lower values may be used. The use of a filter capacitor at the output of the charging device does not modify the process of determining the time t1 _((max)) and the inductance of the inductor L1 _(i).

$\begin{matrix} \begin{matrix} {{t\; 1_{(\max)}} = {{T \times \frac{1}{\frac{1}{A} + B}} =}} \\ {T \times \frac{1}{\begin{matrix} {\frac{1}{\frac{{Vs} + {Vd}}{Ve}} + {\pi \times}} \\ {\sqrt{\frac{1}{2 \times T} \times \frac{C}{\left( {{Vs} + {Vd}} \right) \times {Is}_{i{({moy})}}}} \times \left( {{Ve} - {Vs} - {Vd}} \right)} \end{matrix}}} \\ {= {\frac{1}{50 \times 10^{3}} \times}} \\ {\frac{1}{\begin{matrix} {\frac{1}{\frac{2.5 + 0.3}{{9 \times 3.6} + 2.5}} + {\pi \times}} \\ {\sqrt{\frac{50 \times 10^{3}}{2} \times \frac{2 \times 10^{- 6}}{\left( {3.6 + 0.7} \right) \times 1}} \times \left( {{9 \times 2.5} - 0.7} \right)} \end{matrix}}} \end{matrix} & \left( {{equation}\mspace{14mu} 4} \right) \\ {{{t\; 1_{(\max)}} = {1.0\mspace{14mu}{\mu s}}}\begin{matrix} {{L\; 1_{i{(\max)}}} = {\frac{1}{2} \times \frac{1}{T} \times \frac{\left( {{Ve} - {Vs} - {Vd}} \right)^{2} \times t\; 1^{2}}{\left( {{Vs} + {Vd}} \right) \times {Is}_{i{({moy})}}}}} \\ {= {\frac{1}{2} \times 50 \times 10^{3} \times \frac{\left( {{2.5 \times 9} - 0.7} \right)^{2} \times \left( {1.0 \times 10^{- 6}} \right)^{2}}{\left( {3.6 + 0.7} \right) \times 1}}} \end{matrix}{{L\; 1_{i{(\max)}}} = {2.8\mspace{14mu}{\mu H}}}} & \left( {{equation}\mspace{14mu} 5} \right) \end{matrix}$

Simulations

For this example, two simulation results are illustrated in FIGS. 10 and 11.

First Simulation (FIG. 10):

For the first simulation, the accumulators A_(ij) are charged for the most part to the threshold voltage of 2.5 V and for a single accumulator in stage 7 for example to the voltage of 3.6 V. The charging device 5 is connected in parallel with this accumulator, which has the highest charge voltage, i.e. 3.6 V.

The voltage generator 7 that is simulated is the generator represented in FIG. 6 without the decoupling capacitor C3 _(i).

The first result illustrates the extreme case of operation for which the mean output current must be at least 1 A.

FIG. 10 gives the result of this first simulation and shows the currents in the inductor L1 ₇ in the curve C1, the capacitor C1 ₇ in the curve C2, the diode D1 ₇ in the curve C3, and the accumulator with the highest charge voltage in the curve C4.

As described for the previous operation, the current in the inductor L1 ₇ increases for a time t1. During this time t1, the current does not flow through the accumulator under charge but through the accumulators situated above the latter. The diode D1 ₇ is blocked during this phase.

At the end of the time t1, the value of the current reaches a peak value, which is of the order of 10.7 A in this example. From t1, the current in the inductor decreases and is supplied to the accumulator. The diode D1 ₇ conducts and enables demagnetization of the inductor L1 ₇. The circuit operates in discontinuous conduction mode because the current in the inductor L1 ₇ is cancelled out before each period of operation of the device. The current in the inductor L1 ₇, and thus also through the accumulator to be charged, goes negative when the diode D1 ₇ is blocked, i.e. when the current iD1 ₇ cancels out. This current iL1 ₇ oscillates at a frequency close to the inherent oscillation frequency for the capacitor C1 ₇ and the inductor L1 ₇. This current iL1 ₇ flows through the inductor L1 ₇, the accumulator under charge and the accumulators situated under the stage under charge, the capacitor C1 ₇ and the switch SW3 ₁. The mean current in the capacitor C1 ₇ in the permanent regime is thus zero over an operating period.

The mean output current Is_(7(moy)) is equal to approximately 1.9 A. A minimum mean current of 1 A is complied with whatever the voltage of the charged accumulator and the voltage of the accumulator battery. The current is greater than the desired value mainly because the current in each period T is not equal to 0 as was assumed in dimensioning the parameters of the device. The peak current in the inductor is therefore greater than the value necessary to provide a mean output current of 1 A. The mean current proportional to the peak current in the high inductance squared is therefore seriously impacted (equation 5).

Second Simulation (FIG. 11):

For the second simulation, the accumulators are charged for the most part to the threshold voltage of 3.6 V and for a single accumulator, for example the seventh accumulator, to the voltage of 2.5 V. The charging device 5 is connected in parallel with this accumulator, which has the lowest charge voltage, i.e. 2.5 V. The charging device is therefore connected to the terminals of the seventh stage Et₇.

The second result illustrates the extreme case of operation for which the charging device must operate in discontinuous conduction mode and the current in the inductor L1 _(i) must be zero or positive before each period T of operation.

FIG. 11 shows the result of this second simulation and shows the current in the inductor L1 ₇ in the curve C5, the capacitor C1 ₇ in the curve C6, the diode D1 ₇ in the curve C7, and the accumulator that has the lowest charge voltage in the curve C8.

The circuit functions in discontinuous conduction mode because the current in the inductor L1 _(i) is cancelled out before each period T of operation of the device. Discontinuous conduction operation with a positive or zero current in the inductor L1 _(i) is complied with whatever the voltage of the charged accumulator and the voltage of the accumulator battery.

The mean output current Is_(7(moy)) is equal to approximately 3.4 A. It is greater than the minimum value of 1 A. Via the stages situated above and below the stage under charge, the rms current rises to approximately 1.7 A and the mean current is equal to approximately 330 mA.

Second Embodiment

A second embodiment of the equalization system 2 is described next.

Charging Device

As shown in FIG. 12, the charging device 5 of the second embodiment differs from the first embodiment shown in FIG. 5 in that it includes a second capacitor C2 _(i) the first end of which is connected to the pole v1 of the voltage generator 7 to which the first capacitor C1 _(i) is connected and the second end of which is connected to the pole N_(i) of the stage Et_(i).

This capacitor C2 _(i) provides a return path for some or all of the current supplied by the voltage generator 7.

Voltage Generator

This second embodiment is equally suitable for a voltage generator 7 supplying only a positive voltage pulse, for example.

For the charging device 5 of this second embodiment, two variant embodiments of a voltage generator 7 may be provided, as shown respectively in FIGS. 13 and 14.

First Variant (FIG. 13):

This voltage generator 7 differs from the voltage generator 7 of the first embodiment shown in FIG. 6 because two inductors L3 _(i) and L4 _(i) are implemented in series with the terminals N and P of the battery 1.

This therefore forces the current to flow through the second capacitors C2 _(i) of the charging devices 5 when the switch SW3 _(i) or SW2 _(i) of the voltage generator 7 is closed. The so-called decoupling capacitor C3 _(i) must be connected to the terminals of both switches.

Second Variant (FIG. 14);

The voltage generator 7 shown in FIG. 14 for its part includes a decoupling capacitor C3 _(i), a switch SW2 _(i) and a transformer T1 _(i). This voltage generator 7 has the advantage of minimizing the number of components compared to the voltage generator 7 of the first variant (FIG. 13).

Operation

The operation of the second embodiment associated with the voltage generator 7 shown in FIG. 13 is described below. It is shown in FIGS. 15A, 15B corresponding to the charging of the stage Et₁ and FIG. 16 corresponding more generally to charging a stage Et_(i).

The current in the stage Et₁,Et_(i) and the inductors L3 ₁,L3 _(i) and L4 ₁,L4 _(i) is represented diagrammatically by chain-dotted line arrows, by way of illustration, in that a current in the inductors is necessary for the mean current in the capacitor C3 ₁,C3 _(i) to be zero for a period of operation in the permanent regime. The current is considered direct and thus constant over a period T of operation. In FIG. 16, this current is ignored in the representation of the current iEt_(i)(t) in the stage Et_(i).

Operation is substantially identical to that of the first embodiment. What is additional to the first embodiment is that during the time t1 the current represented by dashed line arrows in FIG. 15A passes through all of the stages situated above and below the stage Et₁,Et_(i) except for stage N. It flows through all the second capacitors C2 _(j) (with j=1 to N) of the charging devices 5. In the ideal case, the current is uniformly divided across all the capacitors. At the end of the time t1 and until the end of the period T of operation (FIG. 15B), current also flows through the stage Et₁, the set of second capacitors C2 _(i) of the charging devices 5 situated below and above the stage Et₁ under charge, the inductance L1 ₁,L1 _(i), the capacitor C1 ₁,C1 _(i), and the switch SW3 ₁,SW3 _(i) (represented in dotted line).

Compared to the first embodiment, this second embodiment has the advantage of educing the rms current in the non-charged stages.

During the time t1 and also from the moment at which the diode D1 ₁, D1 _(i) is blocked up to the end of the period T, the current in the capacitor C1 ₁,C1 _(i) is divided between the capacitors C2 _(i) of the charging devices 5. The closer the stage to the stage under charge, the higher the rms current to flow through this stage.

Moreover, the use of a supplementary switch (not shown) for each charging device connected for example in series between the terminal N₁,N_(i) of the stage Et₁,Et_(i) and the anode of the diode D1 ₁,D1 _(i) enables the rms current in the non-charged stages to be minimized. This switch prevents the capacitor C2 _(i) of the charging device(s) 5 that are not operating from conducting a portion of the current of one or more other charging devices 5 in operation. The rms current through the non-charged stages is in this case theoretically equal to the value of the direct current supplied by the battery.

Dimensioning

The dimensioning of the charging device 5 (FIG. 12) is based on the equations defining its operation as described above. The equations are effective for the associated voltage generator 7 (FIG. 13). For a simple dimensioning, the voltage at the terminals of the capacitor C1 _(i) and the capacitor C2 _(i), if present, is considered to be constant over a period T of operation of the charging device 5.

Equations 1, 2 and 3 for the current (iL1 _(i)) in the inductor L1 _(i) are identical for the second embodiment and the first embodiment, and the same applies to equation 4 for defining the value (t1 _((max))) not to be exceeded of the conduction period t1 and equation 5 for defining the mean output current.

The current, denoted iEt_(i)(t), in a charged stage Et_(i) depends on the number of charging devices operating, the position of the latter relative to the stage Et_(i), and the position of the stage Et_(i) relative to the numbers of stages. Considering the number M of charging devices 5 operating situated below the stage Et_(i) (FIG. 16) and considering the number L of charging devices 5 operating above the stage Et_(i), the current in the stage Et_(i) during the time t1 may be approximated by equation 7. The current in the stage Et_(i) from the moment at which the diode D1 _(i) is blocked to the end of the period T may be approximated by the same equation.

$\begin{matrix} {{{iEt}_{i}(t)} = {{iC}\; 1_{i}(t) \times \left\lbrack {{\frac{i}{N} \times \left( {1 + L + M} \right)} - M} \right\rbrack}} & \left( {{equation}\mspace{14mu} 7} \right) \end{matrix}$

Given that the mean current in the capacitor C1 _(i) is zero over a period of operation in the permanent regime, the mean current in the stage Et_(i) during the time t1 and from the moment at which the diode D1 _(i) is blocked to the end of the period T is therefore zero. Taking into account the current in the inductors L3 _(i) and L4 _(i), the mean current (IEt_(i(moy))) in the charged stage is defined by equation 6.

A decoupling capacitor C3 _(i) is used to implement the voltage generator 7 in this second embodiment. This capacitor C3 _(i) is defined to supply the necessary current to the charging devices 5 with a defined voltage drop at its terminals. The voltage drop at the terminals of the capacitor C3 _(i) as a function of the output current is given by equation 9, K being the number of charging devices 5 operating.

$\begin{matrix} {{\Delta\; V} = {{\frac{1}{2} \times K \times \frac{Ipic}{C\; 1_{i}} \times t\; 1} = {\frac{1}{2} \times K \times \frac{{Ve} - {Vs} - {Vd}}{L\; 1_{i} \times C\; 1_{i}} \times t\; 1^{2}}}} & \left( {{equation}\mspace{14mu} 9} \right) \end{matrix}$

Simulations

The capacitors C2 _(i) have the same values as the capacitors C1 _(i), fixed at 2 μF.

The voltage generator simulated is that shown in FIG. 13.

The value of the two inductors L3 _(i) and L4 _(i) is fixed at 100 μH, which is a typical value in the field of power electronic converters.

A 10 μF decoupling capacitor C3 _(i) is used. This capacitor is defined to supply the necessary current to the charging devices 5 with a certain voltage drop at its terminals. The voltage drop at the terminals of the capacitor as a function of the output current is given by equation 9

The dimensioning of the charging device 5 of the second embodiment is identical to the dimensioning of the charging device of the first embodiment. For this reason, only one simulation result is described in this section in order to confirm that for identical values of the passive components L1 _(i) and C1 _(i) the mean output current of a charging device 5 of the first and second embodiments is identical.

The simulation result illustrates the extreme operating situation in which the device must operate in discontinuous conduction mode and the current in the inductor L1 _(i) must be zero or positive before each period of operation.

FIG. 17 shows the simulation result, showing the currents in the inductor L1 ₇ in the curve C9, the capacitor C1 ₇ in the curve C10, the diode D1 ₇ in the curve C11, and the accumulator that has the lowest charge voltage in the curve C12.

The mean output current Is_(7(moy)) is equal to approximately 3.3 A. It is very close to the mean current value obtained in the first embodiment (approximately 3.4 A). The dimensioning of the first and second embodiments are thus similar.

The mean current in the inductors L4 ₁ and L3 ₁ is equal to approximately 330 mA. The mean current in the stages situated above and below the stage Et₁ under charge is equal to 330 mA. It is identical to the mean current of the non-charged stages of the first embodiment.

The rms current in the stage 8 is equal to 610 mA. The rms current in the stages situated above the stage Et_(i) under charge decreases as the distance of the stage from the stage Et_(i) increases. The rms current in the stage 10 is equal to the direct current that flows through the inductors L3 ₁ and L4 ₁, i.e. 330 mA. The rms current in the stage 6 is of the order of 915 mA. The ms current in the stages situated below the stage Et_(i) under charge decreases as the distance of the stage away from the stage Et_(i) increases. The rms current in the stage 1 is of the order of 360 mA. The rms current in the stages is reduced by a factor of at least 1.8 in the worst case scenario relative to the first embodiment.

Third Embodiment

A third embodiment of the equalization system 2 is described next.

Charging Device

FIG. 18 represents a third embodiment for a charging device 5. This third embodiment differs from the second embodiment in that:

-   -   the second end of the second capacitor C2 _(i) is connected to         the second end of the inductor L1 _(i) and to the pole P_(i) of         the stage,     -   the cathode of the diode D1 _(i) is connected to the first end         of the inductor L1 _(i), and     -   the switch SW1 _(i) is connected to the second end of the         capacitor C1 _(i) and to the first end of the inductor L1 _(i).

In a variant embodiment, a capacitor (not shown) may be added that is configured to filter the ripple on the current coming from the charging device 5 but instead of being connected between the terminal N_(i) and the first end of the switch SW1 _(i), as in the first and second embodiments, in the third embodiment this capacitor is connected between the terminal P_(i) and the anode of the diode D1 _(i).

In the third embodiment, the switch may also be connected to the output v2 of the voltage generator 7 and to the first end of the capacitor C1 _(i).

Voltage Generator

Just like the first and the second embodiments, the third embodiment is suitable for a voltage generator supplying only a positive voltage pulse.

For this third embodiment the voltage generator may be either of the voltage generators shown in the FIGS. 13 and 14 described for the second embodiment.

Operation (FIGS. 16,19A,19B)

The operation of the third embodiment, associated with the voltage generator shown in FIG. 13, is described hereinafter. It is illustrated by FIGS. 19A,19B corresponding to the charging of the stage Et₁ and FIG. 16 corresponding more generally to charging a stage Et_(i).

It is similar to the operation of the second embodiment with the single difference that, during the time t1, the current in the inductor L1 _(i) flows through all the stages situated above and below the stage Et_(i) except for stage 1 and not stage N, as is the case for the second embodiment.

The evolution of the currents in the inductor L1 _(i), the two capacitors C1 _(i) and C2 _(i) and the diode D1 _(i) and the stage Et_(i) in parallel is identical to that of the second embodiment (FIG. 16).

Where the special case of charging the stage Et_(i) is concerned, the evolution of the current in this stage Et_(i) varies from the general case because the current is cancelled out at t1 and also when the diodes are blocked.

Dimensioning

As above, parameterization of the charging device 5 (FIG. 18) is based on equations defining its operation and is effective for the associated voltage generator 7.

Equations 1 to 5 are identical to those for the first and second embodiments.

The current IEt_(i)(t) in a charged stage depends on the number of charging devices operating, their positioning relative to the stage Et_(i), and the position of the stage Et_(i) relative to the numbers of stages. Considering the number M of charging devices operating situated below the stage Et_(i) and considering the number L of charging devices in operation situated above the stage Et_(i), the current in the stage Et_(i) during the time t1 can be approximated by equation 10. The current in the stage Et_(i) from the moment at which the diode D1 _(i) is blocked to the end of the period T may be approximated by the same equation.

$\begin{matrix} {{{iEt}_{i}(t)} = {\frac{{iC}\; 1_{i}(t)}{N} \times \left\lbrack {{\left( {i - 1} \right) \times \left( {L + 1} \right)} - {\left( {N - i + 1} \right) \times M}} \right\rbrack}} & \left( {{equation}\mspace{14mu} 10} \right) \end{matrix}$

Given that the mean current through the capacitor C1 _(i) is zero over a period of operation in the permanent regime, the mean current in the stage Et_(i) during the time t1 and from the moment at which the diode D1 _(i) is blocked to the end of the period T is therefore zero. Considering the current in the inductors L3 _(i) and L4 _(i), the mean current (IEt_(i(moy))) in the charged stage is defined by equation 6.

Simulations

The simulation conditions are identical to those for the second embodiment.

The dimensioning of the charging device 5 of the third embodiment is identical to the dimensioning of the charging device 5 of the first and second embodiments. For this reason, only one simulation result is illustrated in this section in order to confirm that for identical values of the passive components L1 _(i) and C1 _(i) the mean output current of a charging device of these three embodiments is identical.

The simulation result illustrates the extreme situation of operation for which the device must operate in discontinuous conduction mode and the current in the inductor L1 _(i) must be zero or positive before each period of operation T.

FIG. 20 shows the simulation result showing the current in the inductor L1 ₇ in the curve C13, the capacitor C1 ₇ in the curve C14, the diode D1 ₇ in the curve C15 and the accumulator that has the lowest charge voltage in the curve C16.

The mean output current Is_(7(moy)) equal to approximately 3.3 A. It is very close to the mean current obtained for the first and second embodiments (approximately 3.4 A-3.3 A). The dimensioning of the first three embodiments is therefore similar.

The mean current in the inductors L4 ₁ and L3 ₁ is equal to approximately 330 mA. The mean current in the stages situated above and below the stage Et_(i) under charge is equal to 330 mA. It is identical to the mean current of the non-charged stages of the first and second embodiments.

The rms current in the stage 8 is equal to 670 mA. The rms current in the stages situated above the stage Et_(i) under charge decreases as the distance of the stage from the Et_(i) increases. The rms current in the stage 10 is equal to 380 mA. The rms current in the stage 6 is of the order of 890 mA. The rms current in the stages situated below the stage Et_(i) under charge decreases as the distance of the stage from the stage Et_(i) increases. The rms current in the stage 1 is equal to the direct current in the inductors L3 ₁ and L4 ₁, i.e. 330 mA. The rms current in the non-charged stages is decreased by a factor of at least 1.9 in the worst case scenario relative to the first embodiment. The rms current in the non-charged stages is quasi-identical for the third and second embodiments.

Fourth Embodiment

A fourth embodiment of the equalization system 2 is described next.

Charging Device

FIG. 21 shows a fourth embodiment of the charging device 5. This fourth embodiment differs from the second embodiment in that the charging device 5 further includes a second inductor L2 _(i) and a second diode D2 _(i) and in that:

-   -   the second end of the second capacitor C2 _(i) is connected to         the first end of the second inductor L2 _(i),     -   the anode and the cathode of the second diode D2 _(i) are         respectively connected to the pole N_(i) of the stage and to the         first end of the second inductor L2 _(i),     -   the switch SW1 _(i) is connected to the second ends of the two         inductors L1 _(i) and L2 _(i).

The fourth embodiment also differs in that it is adapted to be used with a voltage generator supplying positive and negative voltage pulses, for example.

Variant positions of the switch SW1 _(i) for the fourth embodiment of the charging device are shown in FIGS. 22A and 22B.

In FIG. 22A, the switch SW1 _(i) is connected on its first end to the positive pole v2 of the voltage generator 7 and on its second end to the first end of the first capacitor C1 _(i).

In FIG. 22B, the first end of the switch SW1 _(i) is connected to the second end of the first capacitor C1 _(i) and its second end is connected to the first end of the first inductor L1 _(i).

Moreover, as seen in these figures, a supplementary switch SW11 _(i) must further be connected either to the output v1 of the voltage generator 7 and to the first end of the capacitor C2 _(i) (FIG. 22A) or to the second end of the capacitor C2 _(i) and to the cathode of the second diode D2 _(i) (FIG. 22B).

Voltage Generator

For a charging device 5 of this kind, a first exemplary embodiment of the voltage generator uses a complete bridge with four switches SW2 _(i) to SW5 _(i) and a transformer T1 _(i) (FIG. 23) with its variant shown in FIG. 23′. According to this variant, if a voltage generator 7 is connected to the terminals of the battery 1, the voltage generator 7 may use a transformer T1 _(i) constituted of a primary winding and a plurality of secondary windings. In this example there is shown at the terminals of the primary winding of the transformer T1 _(i) a complete bridge with four switches SW2 _(i) to SW5 ₁. The use of a plurality of secondary windings enables the withstand voltage of the capacitors of the charging devices 5 to be reduced. One secondary winding for the series connection of ten to twelve stages is one suggestion.

A second exemplary embodiment of the voltage generator 7 consists in using a half-bridge with two switches SW2 _(i) and SW3 _(i) and a transformer T1 _(i) the primary of which is connected between the mid-points of the two switches SW2 _(i) and SW3 _(i) and two capacitors C4 _(i) and C5 _(i) (FIG. 24A). The second embodiment has the advantage of a smaller number of switches than the first embodiment and also of avoiding entirely the risk of saturation of the transformer because of an imbalance in the control sequence of the switches thanks to the capacitors C4 _(i) and C5 _(i) in series with the primary winding of the transformer T1 _(i).

A third exemplary embodiment of the voltage generator uses a transformer T1 _(i) with a center-tapped primary and two switches SW2 _(i) and SW3 _(i) (FIG. 24B). The third exemplary embodiment has the advantage of a common reference for controlling the two switches SW2 _(i) and SW3 _(i).

Whichever voltage generator 7 is used, the conduction periods of the switches SW2 _(i) and SW3 _(i) is defined so that each charging device 5 operates in discontinuous conduction mode.

The three exemplary embodiments generate at the output positive and negative voltage pulses.

Operation (FIGS. 25A, 25B and 26)

The operation of the fourth embodiment, associated with the voltage generator 7 shown in FIG. 24A, is described below.

In FIGS. 25A,25B, operation is illustrated only for a positive voltage pulse.

The current in the battery is diagrammatically represented by chain-dotted line arrows by way of illustration, given that the current in the battery is necessary for the mean currents in the capacitors C3 ₁, C4 ₁ and C5 ₁ to be zero over a period of operation in the permanent regime. This current is considered direct and thus constant over a period of operation.

Operational differences compared to the second embodiment with the stage Et₁ under charge are described next.

During the time t1, the current in the inductor L1 ₁ increases proportionately to the voltage applied to its terminals, approximately equal to the voltage at the secondary of the transformer T1 ₁ less the voltage of the stage Et₁ under charge. The current flows only through the stage Et₁. The current in the capacitors C1 ₁ and C2 ₁ of the charging device 5 in operation is equal to the current in the inductor L1 ₁. During this phase, the second diode D2 ₁ of the charging device 5 is conducting. The diode D1 ₁ is blocked with at its terminals a voltage equal at least to the battery voltage. The second inductor L2 ₁ stores energy. The voltage at the terminals at the second inductor L2 ₁ is equal to minus the voltage of the stage Et₁ under charge less the voltage drop with the diode D2 ₁ in the on state.

At the end of the time t1, the switch SW2 ₁ opens. The current in the inductor L1 ₁ reaches a peak value at this time, approximately equal to the voltage applied to the terminals of the inductor when the voltage generator is supplying energy, multiplied by t1 and divided by the value of the inductance.

At the end of the time t1 and up to halfway through the period of operation T/2, the voltage generator does not impose a voltage on the terminals v2 and v1 of the charging device of the stage Et₁. The switches SW3 ₁ and SW2 ₁ are open (FIGS. 25B and 26). The current in the inductor L1 ₁ decreases proportionately to the voltage applied to its terminals, equal at least to the voltage of the accumulator stage Et₁ less the voltage drop with the diode D1 ₁ in the on state. The diode D1 ₁ is on until the current in the stage Et₁ under charge cancels out. The diode D1 ₁ conducts the current in the inductor L1 ₁ and also the current in the inductor L2 ₁. The current in the inductor L2 ₁ is considered constant during this phase in that the impedance of the secondary of the transformer is considered to be much greater than the impedance of the inductors L1 ₁ and L2 ₁. The current in the inductor L2 ₁ is equal to the magnetization current of the transformer. It is denoted lm in FIG. 25. When the diode D1 ₁ is blocked, the current in the inductor L2 ₁ no longer flows through the stage but is equal and opposite the current in the inductor L1 ₁. During this phase, the diode D2 ₁ is blocked with at its terminals a voltage equal at least to the voltage of the stage Et₁.

At the end of the half-period T/2 and until the end of the time T/2 plus the time t1, a negative voltage is applied between the terminals v2 and v1 of the voltage generator. The switch SW3 ₁ is closed and the switch SW2 ₁ is open. Energy is stored in the inductor L2 ₁. The current in the inductor L2 ₁ increases proportionately to the voltage applied to its terminals, approximately equal to the secondary voltage of the transformer T1 ₁ less the voltage of the stage under charge. The current flows only through the stage Et₁ under charge. The current in the capacitors C1 ₁ and C2 ₁ of the charging device in operation is equal to the current in the inductor L2 ₁. During this phase, the diode D1 ₁ conducts. The diode D2 ₁ is blocked with at its terminals a voltage equal at least to the voltage of the battery. The inductor L1 ₁ stores energy. The voltage at the terminals of the inductor L1 ₁ is equal at least to the voltage of the stage Et₁ under charge less the voltage drop with the diode D1 ₁ in the on state.

At the end of the time T/2 plus t1 the switch SW3 ₁ opens. The current in the inductor reaches a peak value at this time, approximately equal to the voltage applied to the terminals of the inductor when the voltage generator is supplying energy, multiplied by t1 and divided by the value of the inductance.

At the end of the time T/2 plus t1 and up to the end of the period T, the voltage generator 7 does not impose a voltage on the terminals v2 and v1 of the charging device 5 of the stage Et₁. The switches SW3 ₁ and SW2 ₁ are open. The current in the inductor L2 ₁ decreases proportionately to the voltage applied to its terminals, equal to at least the voltage of the accumulator stage Et₁ less the voltage drop with the diode D2 ₁ in the on state. The diode D2 ₁ is on until the current in the stage Et₁ under charge cancels out. The diode D2 ₁ conducts the current in the inductor L2 ₁ and also the current in the inductor L1 ₁. The current in the inductor L1 ₁ is equal to the magnetization current (lm) of the transformer. When the diode D2 ₁ is blocked the current in the inductor L1 ₁ no longer flows through the stage but is equal and opposite the current in the inductor L2 ₁. During this phase, the diode D1 ₁ is blocked with at its terminals a voltage equal at least to the voltage of the stage Et₁.

This fourth embodiment has the advantage of imposing a minimum rms current on the stages that are not charged thanks to the fact that the instantaneous current in the capacitors C1 ₁ and C2 ₁ is identical. The rms current is thus equal to the current supplied by the battery 1 considering this current is a direct current.

Dimensioning

As before, the dimensioning of the charging device 5 (FIG. 21) is based on the equations defining its operation and is effective for the associated voltage generator 7.

During the time t1 the current (iL1 _(i)) in the inductor L1 _(i) increases. The current is considered zero in each operating period. The voltage at the secondary of the transformer is denoted VsT1 _(i). The instantaneous sum of the voltage at the terminals of the capacitors C1 _(i) and C2 _(i) is considered zero. Ignoring the voltage drop with the switches SW2 _(i) and SW1 _(i) in the on state and considering the voltage drop Vd with the diode in the on state, the current iL1 _(i)(t) is written:

$\begin{matrix} {{{iL}\; 1_{i}(t)} = {\frac{{{VsT}\; 1_{i}} - {Vs} - {Vd}}{L\; 1_{i}} \times t}} & \left( {{equation}\mspace{14mu} 11} \right) \end{matrix}$

At the end of the time t1, the itch SW2 _(i) opens. The current in the inductor L1 _(i) reaches the peak value of:

$\begin{matrix} {{{iL}\; 1_{i}\left( {t\; 1} \right)} = {{Ipic} = {\frac{{{VsT}\; 1_{i}} - {Vs} - {Vd}}{L\; 1_{i}} \times t\; 1}}} & \left( {{equation}\mspace{14mu} 12} \right) \end{matrix}$

At the end of the time t1 and until cancellation of the current in the inductor L1 _(i), the diode D1 _(i) conducts. The current in the inductor L1 _(i) decreases in accordance with the following law.

$\begin{matrix} {{{iL}\; 1_{i}(t)} = {{{- \frac{{Vs} + {Vd}}{L\; 1_{i}}} \times t} + {Ipic}}} & \left( {{equation}\mspace{14mu} 13} \right) \end{matrix}$

The current in the inductor L2 _(i) is governed between T/2 and T by the same equations as the current in the inductor L1 _(i) between the time 0 and half the period of operation T/2.

From equation 13, the value (t1 _((max))) not to be exceeded of the conduction period t1 for the device to operate in discontinuous conduction mode can be defined. The current in the inductors L1 _(i) and L2 _(i) cancels out before each half-period of operation of the device. To limit the ripple on the output current, it is possible to cancel out the current in the inductors in the vicinity of each period of operation. In the present situation, see FIG. 26, the time t1 _((max)), is determined considering the current in the inductors L1 _(i) and L2 _(i) to be cancelled at T/2. To consider the worst case scenario, the time t1 _((max)) is to be evaluated for the maximum input voltage VsT1 _(i) and the minimum output voltage Vs. Moreover, the voltage drops of the diodes may be ignored in order to consider the worst case scenario.

$\begin{matrix} {{t\; 1_{(\max)}} = {\frac{T}{2} \times \frac{1}{\frac{{{VsT}\; 1_{i}} - {Vs} - {Vd}}{{Vs} + {Vd}} + 1}}} & \left( {{equation}\mspace{14mu} 14} \right) \end{matrix}$

The output current is_(i)(t) of the charging device is equal to the sum of the currents in the inductors L1 _(i) and L2 _(i). The mean output current (Is_(i(moy))) is calculated from equation 15 ignoring lm and the currents iL₁ and iL₂ during the periods when the diodes D1 _(i) and D2 ₁, respectively, conduct. It is further assumed that the inductors L1 _(i) and L2 _(i) are identical.

$\begin{matrix} {{Is}_{i{({moy})}} = {{\frac{Ipic}{T} \times \left\lbrack {t_{1} + \frac{{Ipic} \times L\; 1_{i}}{\left( {{Vs} + {Vd}} \right)}} \right\rbrack} = {\frac{t\; 1^{2}}{T \times L\; 1_{i}} \times \frac{{VsT}\; 1_{i} \times \left( {{{VsT}\; 1_{i}} - {Vs} - {Vd}} \right)}{{Vs} + {Vd}}}}} & \left( {{equation}\mspace{14mu} 15} \right) \end{matrix}$

The current, denoted iEt_(i)(t), in a charged stage i depends only on the number of charging devices operating for this embodiment. Considering the number K of charging devices operating and the transformation ratio m of the transformer T1 _(i) (m=v2/v1), the mean value (IEt_(i(moy))) of the current in the charged stage is obtained from equation 16. The currents in the inductors L1 _(i) and L2 _(i) when the diodes D1 _(i) and D2 _(i), respectively, conduct is not considered

$\begin{matrix} {{IEt}_{i{({moy})}} = {\frac{1}{T} \times {Ipic} \times \left\lbrack {{t_{1} \times \left( {1 - \frac{K \times m}{2}} \right)} + \frac{{Ipic} \times L\; 1_{i}}{\left( {{Vs} + {Vd}} \right)}} \right\rbrack}} & \left( {{equation}\mspace{14mu} 16} \right) \end{matrix}$

EXAMPLE

For the fourth embodiment of the charging device 5 associated with the voltage generator 7 from FIG. 21, the transformation ratio m of the transformer T1 _(i) is fixed at 2 in order to maintain at the secondary of the transformer T1 _(i) a voltage equal to the battery voltage.

The hypotheses are the same as those of the first embodiment but the frequency is 25 kHz.

The time t1 _((max)) is calculated using the minimum voltage drop of the diodes and the maximum input voltage and the minimum output voltage of the charging device. The maximum values of the inductors L1 _(i(max)) and L2 _(i(max)) are for their part calculated using the maximum voltage drop of the diodes and the minimum input voltage and the maximum output voltage of the charging device. The time t1 _((max)) is used to calculate the value of the inductor L1 _(i) in order to minimize the peak current and the rms current in the inductor, the switches and the diodes. For robustness, lower values may be used.

$\begin{matrix} {\begin{matrix} {{t\; 1_{(\max)}} = {\frac{T}{2} \times \frac{1}{\frac{{{VsT}\; 1_{i}} - {Vs} - {Vd}}{{Vs} + {Vd}} + 1}}} \\ {= {\frac{1}{2 \times 25 \times 10^{3}} \times \frac{1}{\frac{{3.6 \times 9} - 0.3}{2.5 + 0.3} + 1}}} \end{matrix}{{t\; 1_{(\max)}} = {1.60\mspace{14mu}{\mu s}}}} & \left( {{equation}\mspace{14mu} 14} \right) \\ {\begin{matrix} {{L\; 1_{i{(\max)}}} = {{L\; 2_{i{(\max)}}} = {\frac{t\; 1^{2}}{T \times {Is}_{i{({moy})}}} \times}}} \\ {\frac{{VsT}\; 1_{i} \times \left( {{{VsT}\; 1_{i}} - {Vs} - {Vd}} \right)}{{Vs} + {Vd}}} \\ {= {25 \times 10^{3} \times \left( {1.60 \times 10^{- 6}} \right)^{2} \times}} \\ {\frac{\left( {{2.5 \times 9} + 3.6} \right) \times \left( {{2.5 \times 9} - 0.7} \right)}{3.6 + 0.7}} \end{matrix}{{L\; 1_{i{(\max)}}} = {{L\; 2_{i{(\max)}}} = {8.5\mspace{14mu}{\mu H}}}}} & \left( {{equation}\mspace{14mu} 15} \right) \end{matrix}$

Simulations

The value of the inductors L1 _(i) and L2 _(i) is fixed at 8.5 μH. The value of the blocking capacitors C1 _(i) and C2 _(i) is fixed at 2 μF.

In this fourth embodiment, a typical operating frequency (F) of 25 kHz is assumed, i.e. T=1/F=40 μs.

The voltage generator 7 simulated is that shown in FIG. 24A. It supplies a positive voltage pulse when the switch SW2 _(i) is closed and the switch SW3 _(i) is open. It supplies a negative voltage pulse as long as the switch SW3 _(i) is closed and the switch SW2 _(i) is open.

When the switches SW2 _(i) and SW3 _(i) are in the open state, the voltage at the terminals of the primary of the transformer T1 _(i) is not fixed. The conduction period of the switches SW2 _(i) and SW3 _(i) is fixed at 1.6 μs. A diode is implemented in parallel with each switch in order to enable demagnetization of the transformer primary also. The capacitors C3 _(i), C4 _(i) and C5 _(i) are fixed at 10 μF in this example. As for the third embodiment, these capacitors are defined to supply the necessary current to the primary of the transformer T1 _(i) with a certain voltage drop at its terminals. The voltage drop at the terminals of the capacitor is given as a function of the output current by equation 9.

An inductor L5 connected in series with the battery and the input of the voltage generator is used to filter the ripple on the current from the battery.

First Simulation (FIG. 27):

The first result illustrates the extreme case of operation for which the mean output current must be at least 1 A.

FIG. 27 shows the simulation result showing the currents in the inductors L1 ₇ and L2 ₇ in the curves C17 and C18, respectively, the capacitor C1 ₇ in the curve C19, the diode D1 ₇ in the curve C20, the output of the charging device in the curve C21 and the accumulator having the highest charge voltage in the curve C22.

The operation observed is similar to the operation described in the previous section concerning the fourth embodiment.

The mean output current Is_(7(moy)) is equal to approximately 1.1 A. A minimum mean current of 1 A is complied with whatever the voltage of the charged accumulator and the voltage of the accumulator battery.

The circuit operates in discontinuous conduction mode with the current in the inductors L1 _(i) and L2 _(i) cancelled out before each half-period of operation of the device, as is required.

Second Simulation (FIG. 28):

The second result illustrates the extreme case of operation for which the device must operate in discontinuous conduction mode, i.e. the current flowing in the inductors L1 _(i) and L2 _(i) must cancel out before each half-period of operation T/2 of the device.

FIG. 28 shows the simulation result showing the currents in the inductors L1 ₇ and L2 ₇ in the curves C23 and C24, respectively, the capacitor C1 ₇ in the curve C25, the diode D1 ₇ in the curve C26, the output of the charging device in the curve C27 and the accumulator having the lowest charge voltage in the curve C28.

The circuit operates in discontinuous conduction mode and the current flowing in the inductors L1 _(i) and L2 _(i) is cancelled out as required before each half-period of operation of the device. Operation in discontinuous conduction mode is complied with whatever the voltage of the charged accumulator and the voltage of the accumulator battery.

The mean output current Is_(7(moy)) is equal to approximately 2.8 A. It is greater than the minimum value of 1 A. The direct current in the battery is equal to approximately 260 mA. The rms current in the non-charged stages is equal to the direct current supplied by the battery.

Accordingly, whichever embodiment is used, effective equalization of the charging of the accumulator battery 1 is obtained. 

The invention claimed is:
 1. An apparatus comprising a battery equalization system, said battery equalization system comprising two stages of accumulators placed in series, each stage having a negative pole and a positive pole, each stage comprising an accumulator placed between said negative pole and said positive pole, a voltage generator comprising a positive terminal and a negative terminal, for each stage of accumulators, an associated charging device powered by said voltage generator, said associated charging device comprising an inductor, a first capacitor having a first end and a second end, said first end being connected to a terminal of said voltage generator, a diode having an anode and a cathode, said diode being connected by said anode to said negative pole of said associated stage of accumulators and connected to an end of said inductor to allow, when said diode is forward biased, a flow of a charging current through said associated stage of accumulators, said associated diode and said associated inductor, and a switch, one end of which is linked to an end of said inductor so that said switch, said inductor and said first capacitor of said associated charging device are connected in series between a terminal of said voltage generator and a pole of said associated stage of accumulators, and a control device configured to, during a conduction time interval, to apply a voltage variation between said terminals of said voltage generator and to close a switch of a charging device associated with a stage of accumulators to be charged such that said inductor and said switch of said charging device are traversed by an increasing power supply current originating from said voltage generator and such that said inductor stores energy, and after said conduction time interval, to interrupt said power supply current through said inductor and allow transfer of the energy stored in said inductor to said associated stage of accumulators by flow of a decreasing charging current through said associated inductor and said associated diode.
 2. The apparatus of claim 1, wherein said voltage generator is connected to a predefined number of charging devices.
 3. The apparatus of claim 1, wherein said first capacitor comprises a first end connected to said positive terminal of said voltage generator.
 4. The apparatus of claim 3, wherein a second end of said first capacitor is connected to a first end of said inductor, a cathode of said diode is connected to the second end of the first capacitor, and the switch of the charging device is connected by a first end to a second end of the inductor and by a second end to the positive pole of the associated stage of accumulators.
 5. The apparatus of claim 3, wherein said charging device includes a first capacitor, a first side of which is connected to said positive terminal of said voltage generator and a second side of which is connected to a first end of said inductor, a second capacitor, a first side of which is connected to said negative terminal of said voltage generator and a second side of which is connected to said negative pole of said associated stage of accumulators or to a second end of said inductor and to said positive pole of said associated stage of accumulators, and wherein said diode is connected, by said anode thereof, to said negative pole of said associated stage of accumulators, and wherein said diode is connected, by said cathode thereof, to said first end of said inductor, and wherein said switch of said charging device is connected by one end to said inductor and by another end to said positive pole of said associated stage of accumulators or to said first capacitor.
 6. The apparatus of claim 1, wherein said diode is connected by said anode to said negative pole of said associated stage of accumulators, and wherein said diode is connected by a cathode thereof to a first end of said inductor.
 7. The apparatus of claim 1, wherein said control device is further configured to interrupt said power supply current through said inductor by imposing a zero voltage across said terminals of said voltage generator.
 8. The apparatus of claim 1, wherein said control device is configured to control a rate of closing of said switch of a charging device associated with a stage of accumulators to be charged in response to application, beforehand, of a voltage to said poles of said voltage generator feeding said charging device.
 9. The apparatus of claim 1, wherein said voltage generator comprises at least one switch controlled by said control device, and wherein said voltage generator comprises a decoupling capacitor.
 10. The apparatus of claim 9, wherein said voltage generator comprises a configuration selected from the group consisting of a first configuration and a second configuration, wherein said first configuration comprises two controlled switches and two inductors, and wherein said second configuration comprises a switch and a transformer.
 11. The apparatus of claim 9, wherein said control device is configured to close and to open said controlled switch of said voltage generator device with a conduction time interval and an opening time interval that are constant during a charging phase.
 12. The apparatus of claim 11, wherein said conduction time interval is calculated such that said charging device operates in discontinuous conduction mode independently of voltage levels of said associated stage of accumulators and of a battery during said charging phase.
 13. The apparatus of claim 1, further comprising a battery including an elementary module, said elementary module comprising terminals and a plurality of stages of accumulators in series, and a supplementary charging device at said terminals.
 14. The apparatus of claim 13, wherein said battery comprises a plurality of elementary modules disposed in series, and wherein said equalization system includes a supplementary charging device at terminals of a predetermined number of said elementary modules.
 15. The apparatus of claim 1, further comprising a charging device for a battery stage of accumulators, wherein said equalization system is operably coupled to said charging device. 